Changes in mortality associated with temperature-sensitive diseases in Japan during the COVID-19 pandemic

In a recent study published in PLOS ONE, researchers assessed the impact of the coronavirus disease 2019 (COVID-19) pandemic on temperature-sensitive disease mortality.

Cardiovascular and respiratory illnesses caused 345,476 and 172,727 deaths during the COVID-19 pandemic in 2020, respectively. Compared to 2019 (pre-COVID-19), this represented a considerable decrease of 5,029 and 20,507, respectively. This indicates that examining the alterations in the number of cardiovascular and respiratory mortality caused by behavioral changes will be crucial in researching various issues, including climate change, public health, and the aging population.

About the study

In the present study, researchers assessed the variations in temperature-sensitive disease mortality during the COVID-19 pandemic in Japan.

The three largest cities that were the focus of this study were Sapporo City (Sapporo), Osaka City (Osaka), and Tokyo 23 wards (Tokyo), which have a population of 1.97 million, 2.75 million, and 9.71 million, respectively, in 2021. These teams were assessed because they exhibited different climates. For example, in August, the mean ambient temperatures were 22.3 °C in Sapporo, 29 °C in Osaka, and 26.9 °C in Tokyo, while the mean temperatures in December were 0.9 °C in Sapporo, 17.1 °C in Osaka, and 15.8 °C in Tokyo.

The monthly death toll for each cause of death is made available on the Japanese government's "e-Stat" portal website. Due to the statistical reports produced by the Japanese Ministry of Health, Labour, and Welfare (MHLW), the figures were highly reliable. Heart failure (HF), intracerebral hemorrhage (ICH), cerebral infarction (CI), respiratory diseases (Resp), cardiac arrhythmia and conduction disorders (CACD), and ischemic heart disease (IHD) were the seven illnesses examined in this study. The age-adjusted mortality rate (MR_adj) was applied to the population by age group to account for yearly variations and city variances.

The available data collected and provided by the Japan Meteorological Agency (JMA) served as the source for the monthly mean temperature (MMT). The Automated Meteorological Data Acquisition System (AMeDAS), a network of numerous weather stations, has been installed by the JMA all across Japan. For the analysis, the corresponding AMeDAS in Sapporo City, Osaka City, and Ota Ward in Tokyo were selected. The team estimated the outdoor air temperature approximately 1.5 m above the ground.

Furthermore, the duration of the COVID-19 pandemic was considered to be from March to December 2020, and the months between 2010 and 2019 were compared with those in the pre-pandemic era.

Results

For CACD, CI, HF, IHD, ICH, and Resp, the MR_adj steadily climbed in the winter and declined in the summer from January 2019 to December 2020. Numerous studies have documented an increase in respiratory and cardiovascular mortality in older persons during the winter. In March 2020, COVID-19 started to spread in Japan. The overlap between the declining period and the COVID-19 spreading period appears to be coincidental since the reduction in MR_adj between spring and summer was observed in 2020 as well as in the preCOVID-19 period.

With declining annual MMT in all cities, MR_adj increased with statistically significant associations for May IHD in Osaka and Sapporo, May CI in Osaka and Tokyo, and May Resp in Osaka and Tokyo. This indicated that the MMT for the majority of diseases in these three cities had a high contribution rate for the MR_adj in May.

Despite having a more temperate environment and a greater MMT than Sapporo, both Tokyo and Osaka demonstrated a significantly negative response of MR_adj towards MMT in May. This could be because the optimum temperature where temperature-sensitive mortality is at its lowest throughout the year was found to vary by region. The team noted that the August MR_adj of the assessed cities had a positive response to MMT. This observation was also made in Sapporo whose August MMT was considerably smaller than that in Tokyo and Osaka. Therefore, if the temperature in a given year was greater or lower than the city's climatologically ideal temperature, disease mortality increased.

In Sapporo, it was anticipated that an additional 324 to 980 persons would die between April and December 2020. This was equivalent to 1.19 to 1.56 times the number of recorded deaths. On the other hand, 651 to 2653 deaths were predicted in Tokyo between April and December 2020, which equated to 1.10 to 1.39 times the actual number of deaths reported. In Osaka, an estimated 235 to 1,343 deaths occurred, which is 1.08 to 1.48 times the number of deaths reported over the same period. In the absence of the pandemic, in Sapporo, Osaka, and Tokyo, death rates corresponding to temperature-sensitive diseases would have increased by 19% to 56%, 8% to 48%, and 10% to 39%.

Conclusion

Overall, the study utilized the data obtained from three major Japanese cities of Sapporo, Tokyo, and Osaka, to assess the mortality changes related to temperature-sensitive respiratory and cardiovascular diseases during the COVID-19 pandemic.

Written by

Bhavana Kunkalikar

Bhavana Kunkalikar is a medical writer based in Goa, India. Her academic background is in Pharmaceutical sciences and she holds a Bachelor's degree in Pharmacy. Her educational background allowed her to foster an interest in anatomical and physiological sciences. Her college project work based on ‘The manifestations and causes of sickle cell anemia’ formed the stepping stone to a life-long fascination with human pathophysiology.